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templates

Resource template functionality.

ResourceSecurity dataclass

Security policy applied to extracted resource template parameters.

These checks run after :meth:~mcp.shared.uri_template.UriTemplate.match has extracted and decoded parameter values. They catch path-traversal and absolute-path injection regardless of how the value was encoded in the URI (literal, %2F, %5C, %2E%2E).

Example::

# Opt out for a parameter that legitimately contains ..
@mcp.resource(
    "git://diff/{+range}",
    security=ResourceSecurity(exempt_params={"range"}),
)
def git_diff(range: str) -> str: ...
Source code in src/mcp/server/mcpserver/resources/templates.py 
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@dataclass(frozen=True)
class ResourceSecurity:
    """Security policy applied to extracted resource template parameters.

    These checks run after :meth:`~mcp.shared.uri_template.UriTemplate.match`
    has extracted and decoded parameter values. They catch path-traversal
    and absolute-path injection regardless of how the value was encoded in
    the URI (literal, ``%2F``, ``%5C``, ``%2E%2E``).

    Example::

        # Opt out for a parameter that legitimately contains ..
        @mcp.resource(
            "git://diff/{+range}",
            security=ResourceSecurity(exempt_params={"range"}),
        )
        def git_diff(range: str) -> str: ...
    """

    reject_path_traversal: bool = True
    """Reject values containing ``..`` as a path component."""

    reject_absolute_paths: bool = True
    """Reject values that look like absolute filesystem paths."""

    reject_null_bytes: bool = True
    """Reject values containing NUL (``\\x00``). Null bytes defeat string
    comparisons (``"..\\x00" != ".."``) and can cause truncation in C
    extensions or subprocess calls."""

    exempt_params: Set[str] = field(default_factory=frozenset[str])
    """Parameter names to skip all checks for."""

    def validate(self, params: Mapping[str, str | list[str]]) -> str | None:
        """Check all parameter values against the configured policy.

        Args:
            params: Extracted template parameters. List values (from
                explode variables) are checked element-wise.

        Returns:
            The name of the first parameter that fails, or ``None`` if
            all values pass.
        """
        for name, value in params.items():
            if name in self.exempt_params:
                continue
            values = value if isinstance(value, list) else [value]
            for v in values:
                if self.reject_null_bytes and "\0" in v:
                    return name
                if self.reject_path_traversal and contains_path_traversal(v):
                    return name
                if self.reject_absolute_paths and is_absolute_path(v):
                    return name
        return None

reject_path_traversal class-attribute instance-attribute 

reject_path_traversal: bool = True

Reject values containing .. as a path component.

reject_absolute_paths class-attribute instance-attribute 

reject_absolute_paths: bool = True

Reject values that look like absolute filesystem paths.

reject_null_bytes class-attribute instance-attribute 

reject_null_bytes: bool = True

Reject values containing NUL (\x00). Null bytes defeat string comparisons ("..\x00" != "..") and can cause truncation in C extensions or subprocess calls.

exempt_params class-attribute instance-attribute 

exempt_params: Set[str] = field(
    default_factory=frozenset[str]
)

Parameter names to skip all checks for.

validate 

validate(
    params: Mapping[str, str | list[str]],
) -> str | None

Check all parameter values against the configured policy.

Parameters:

Name Type Description Default
params Mapping[str, str | list[str]]

Extracted template parameters. List values (from explode variables) are checked element-wise.

 required

Returns:

Type Description
str | None

The name of the first parameter that fails, or None if
str | None

all values pass.
Source code in src/mcp/server/mcpserver/resources/templates.py 
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def validate(self, params: Mapping[str, str | list[str]]) -> str | None:
    """Check all parameter values against the configured policy.

    Args:
        params: Extracted template parameters. List values (from
            explode variables) are checked element-wise.

    Returns:
        The name of the first parameter that fails, or ``None`` if
        all values pass.
    """
    for name, value in params.items():
        if name in self.exempt_params:
            continue
        values = value if isinstance(value, list) else [value]
        for v in values:
            if self.reject_null_bytes and "\0" in v:
                return name
            if self.reject_path_traversal and contains_path_traversal(v):
                return name
            if self.reject_absolute_paths and is_absolute_path(v):
                return name
    return None

DEFAULT_RESOURCE_SECURITY module-attribute 

DEFAULT_RESOURCE_SECURITY = ResourceSecurity()

Secure-by-default policy: traversal, absolute paths, and null bytes rejected.

ResourceSecurityError

Bases: ValueError

Raised when an extracted parameter fails :class:ResourceSecurity checks.

Distinct from a simple None non-match so that template iteration can stop at the first security rejection rather than falling through to a later, possibly more permissive, template.

Source code in src/mcp/server/mcpserver/resources/templates.py 
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class ResourceSecurityError(ValueError):
    """Raised when an extracted parameter fails :class:`ResourceSecurity` checks.

    Distinct from a simple ``None`` non-match so that template
    iteration can stop at the first security rejection rather than
    falling through to a later, possibly more permissive, template.
    """

    def __init__(self, template: str, param: str) -> None:
        super().__init__(f"Parameter {param!r} of template {template!r} failed security validation")
        self.template = template
        self.param = param

ResourceTemplate

Bases: BaseModel

A template for dynamically creating resources.

Source code in src/mcp/server/mcpserver/resources/templates.py 
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class ResourceTemplate(BaseModel):
    """A template for dynamically creating resources."""

    uri_template: str = Field(description="URI template with parameters (e.g. weather://{city}/current)")
    name: str = Field(description="Name of the resource")
    title: str | None = Field(description="Human-readable title of the resource", default=None)
    description: str | None = Field(description="Description of what the resource does")
    mime_type: str = Field(default="text/plain", description="MIME type of the resource content")
    icons: list[Icon] | None = Field(default=None, description="Optional list of icons for the resource template")
    annotations: Annotations | None = Field(default=None, description="Optional annotations for the resource template")
    meta: dict[str, Any] | None = Field(default=None, description="Optional metadata for this resource template")
    fn: Callable[..., Any] = Field(exclude=True)
    parameters: dict[str, Any] = Field(description="JSON schema for function parameters")
    context_kwarg: str | None = Field(None, description="Name of the kwarg that should receive context")
    parsed_template: UriTemplate = Field(exclude=True, description="Parsed RFC 6570 template")
    security: ResourceSecurity = Field(exclude=True, description="Path-safety policy for extracted parameters")

    @classmethod
    def from_function(
        cls,
        fn: Callable[..., Any],
        uri_template: str,
        name: str | None = None,
        title: str | None = None,
        description: str | None = None,
        mime_type: str | None = None,
        icons: list[Icon] | None = None,
        annotations: Annotations | None = None,
        meta: dict[str, Any] | None = None,
        context_kwarg: str | None = None,
        security: ResourceSecurity = DEFAULT_RESOURCE_SECURITY,
    ) -> ResourceTemplate:
        """Create a template from a function.

        Raises:
            InvalidUriTemplate: If ``uri_template`` is malformed or uses
                unsupported RFC 6570 features.
        """
        func_name = name or fn.__name__
        if func_name == "<lambda>":
            raise ValueError("You must provide a name for lambda functions")  # pragma: no cover

        parsed = UriTemplate.parse(uri_template)

        # Find context parameter if it exists
        if context_kwarg is None:  # pragma: no branch
            context_kwarg = find_context_parameter(fn)

        # Get schema from func_metadata, excluding context parameter
        func_arg_metadata = func_metadata(
            fn,
            skip_names=[context_kwarg] if context_kwarg is not None else [],
        )
        parameters = func_arg_metadata.arg_model.model_json_schema()

        # ensure the arguments are properly cast
        fn = validate_call(fn)

        return cls(
            uri_template=uri_template,
            name=func_name,
            title=title,
            description=description or fn.__doc__ or "",
            mime_type=mime_type or "text/plain",
            icons=icons,
            annotations=annotations,
            meta=meta,
            fn=fn,
            parameters=parameters,
            context_kwarg=context_kwarg,
            parsed_template=parsed,
            security=security,
        )

    def matches(self, uri: str) -> dict[str, str | list[str]] | None:
        """Check if a URI matches this template and extract parameters.

        Delegates to :meth:`UriTemplate.match` for RFC 6570 extraction,
        then applies this template's :class:`ResourceSecurity` policy
        (path traversal, absolute paths).

        Returns:
            Extracted parameters on success, or ``None`` if the URI
            doesn't match the template.

        Raises:
            ResourceSecurityError: If the URI matches but an extracted
                parameter fails security validation. Raising (rather
                than returning ``None``) prevents the resource manager
                from silently falling through to a later, possibly more
                permissive, template.
        """
        params = self.parsed_template.match(uri)
        if params is None:
            return None
        failed = self.security.validate(params)
        if failed is not None:
            raise ResourceSecurityError(self.uri_template, failed)
        return params

    async def create_resource(
        self,
        uri: str,
        params: dict[str, Any],
        context: Context[LifespanContextT, RequestT],
    ) -> Resource | InputRequiredResult:
        """Create a resource from the template with the given parameters.

        An `InputRequiredResult` returned by the template function is passed
        through unchanged (the 2026-07-28 multi-round-trip flow); the retry's
        answers arrive on `ctx.input_responses`, with `ctx.request_state`
        carrying the echoed opaque state.

        Raises:
            ResourceError: If creating the resource fails.
        """
        try:
            # Add context to params if needed
            params = inject_context(self.fn, params, context, self.context_kwarg)

            fn = self.fn
            if is_async_callable(fn):
                result = await fn(**params)
            else:
                result = await anyio.to_thread.run_sync(functools.partial(self.fn, **params))

            if isinstance(result, InputRequiredResult):
                return result

            return FunctionResource(
                uri=uri,  # type: ignore
                name=self.name,
                title=self.title,
                description=self.description,
                mime_type=self.mime_type,
                icons=self.icons,
                annotations=self.annotations,
                meta=self.meta,
                fn=lambda: result,  # Capture result in closure
            )
        except (ResourceError, MCPError):
            raise
        except Exception as exc:
            logger.exception(f"Error creating resource from template {uri}")
            raise ResourceError(f"Error creating resource from template {uri}") from exc

from_function classmethod 

from_function(
    fn: Callable[..., Any],
    uri_template: str,
    name: str | None = None,
    title: str | None = None,
    description: str | None = None,
    mime_type: str | None = None,
    icons: list[Icon] | None = None,
    annotations: Annotations | None = None,
    meta: dict[str, Any] | None = None,
    context_kwarg: str | None = None,
    security: ResourceSecurity = DEFAULT_RESOURCE_SECURITY,
) -> ResourceTemplate

Create a template from a function.

Raises:

Type Description
InvalidUriTemplate

If uri_template is malformed or uses unsupported RFC 6570 features.
Source code in src/mcp/server/mcpserver/resources/templates.py 
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@classmethod
def from_function(
    cls,
    fn: Callable[..., Any],
    uri_template: str,
    name: str | None = None,
    title: str | None = None,
    description: str | None = None,
    mime_type: str | None = None,
    icons: list[Icon] | None = None,
    annotations: Annotations | None = None,
    meta: dict[str, Any] | None = None,
    context_kwarg: str | None = None,
    security: ResourceSecurity = DEFAULT_RESOURCE_SECURITY,
) -> ResourceTemplate:
    """Create a template from a function.

    Raises:
        InvalidUriTemplate: If ``uri_template`` is malformed or uses
            unsupported RFC 6570 features.
    """
    func_name = name or fn.__name__
    if func_name == "<lambda>":
        raise ValueError("You must provide a name for lambda functions")  # pragma: no cover

    parsed = UriTemplate.parse(uri_template)

    # Find context parameter if it exists
    if context_kwarg is None:  # pragma: no branch
        context_kwarg = find_context_parameter(fn)

    # Get schema from func_metadata, excluding context parameter
    func_arg_metadata = func_metadata(
        fn,
        skip_names=[context_kwarg] if context_kwarg is not None else [],
    )
    parameters = func_arg_metadata.arg_model.model_json_schema()

    # ensure the arguments are properly cast
    fn = validate_call(fn)

    return cls(
        uri_template=uri_template,
        name=func_name,
        title=title,
        description=description or fn.__doc__ or "",
        mime_type=mime_type or "text/plain",
        icons=icons,
        annotations=annotations,
        meta=meta,
        fn=fn,
        parameters=parameters,
        context_kwarg=context_kwarg,
        parsed_template=parsed,
        security=security,
    )

matches 

matches(uri: str) -> dict[str, str | list[str]] | None

Check if a URI matches this template and extract parameters.

Delegates to :meth:UriTemplate.match for RFC 6570 extraction, then applies this template's :class:ResourceSecurity policy (path traversal, absolute paths).

Returns:

Type Description
dict[str, str | list[str]] | None

Extracted parameters on success, or None if the URI
dict[str, str | list[str]] | None

doesn't match the template.

Raises:

Type Description
ResourceSecurityError

If the URI matches but an extracted parameter fails security validation. Raising (rather than returning None) prevents the resource manager from silently falling through to a later, possibly more permissive, template.
Source code in src/mcp/server/mcpserver/resources/templates.py 
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def matches(self, uri: str) -> dict[str, str | list[str]] | None:
    """Check if a URI matches this template and extract parameters.

    Delegates to :meth:`UriTemplate.match` for RFC 6570 extraction,
    then applies this template's :class:`ResourceSecurity` policy
    (path traversal, absolute paths).

    Returns:
        Extracted parameters on success, or ``None`` if the URI
        doesn't match the template.

    Raises:
        ResourceSecurityError: If the URI matches but an extracted
            parameter fails security validation. Raising (rather
            than returning ``None``) prevents the resource manager
            from silently falling through to a later, possibly more
            permissive, template.
    """
    params = self.parsed_template.match(uri)
    if params is None:
        return None
    failed = self.security.validate(params)
    if failed is not None:
        raise ResourceSecurityError(self.uri_template, failed)
    return params

create_resource async 

create_resource(
    uri: str,
    params: dict[str, Any],
    context: Context[LifespanContextT, RequestT],
) -> Resource | InputRequiredResult

Create a resource from the template with the given parameters.

An InputRequiredResult returned by the template function is passed through unchanged (the 2026-07-28 multi-round-trip flow); the retry's answers arrive on ctx.input_responses, with ctx.request_state carrying the echoed opaque state.

Raises:

Type Description
ResourceError

If creating the resource fails.
Source code in src/mcp/server/mcpserver/resources/templates.py 
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async def create_resource(
    self,
    uri: str,
    params: dict[str, Any],
    context: Context[LifespanContextT, RequestT],
) -> Resource | InputRequiredResult:
    """Create a resource from the template with the given parameters.

    An `InputRequiredResult` returned by the template function is passed
    through unchanged (the 2026-07-28 multi-round-trip flow); the retry's
    answers arrive on `ctx.input_responses`, with `ctx.request_state`
    carrying the echoed opaque state.

    Raises:
        ResourceError: If creating the resource fails.
    """
    try:
        # Add context to params if needed
        params = inject_context(self.fn, params, context, self.context_kwarg)

        fn = self.fn
        if is_async_callable(fn):
            result = await fn(**params)
        else:
            result = await anyio.to_thread.run_sync(functools.partial(self.fn, **params))

        if isinstance(result, InputRequiredResult):
            return result

        return FunctionResource(
            uri=uri,  # type: ignore
            name=self.name,
            title=self.title,
            description=self.description,
            mime_type=self.mime_type,
            icons=self.icons,
            annotations=self.annotations,
            meta=self.meta,
            fn=lambda: result,  # Capture result in closure
        )
    except (ResourceError, MCPError):
        raise
    except Exception as exc:
        logger.exception(f"Error creating resource from template {uri}")
        raise ResourceError(f"Error creating resource from template {uri}") from exc